Numerous devices have previously been developed for the transdermal delivery of drugs and other medicinal compounds utilizing microneedle assemblies. Microneedles have the advantage of causing less pain to the patient as compared to larger conventional needles. In addition, conventional subcutaneous (often intra-muscular) delivery of drugs via a needle acts to deliver large amounts of a drug at one time, thereby often creating a spike in the bioavailability of the drug. For drugs with certain metabolic profiles this is not a significant problem. However, many drugs benefit from having a steady state concentration in the patient's blood stream, a well-known example of such a drug is insulin. Transdermal drug delivery devices are technically capable of slowly administering drugs at a constant rate over an extended period of time. Thus, transdermal drug delivery devices offer several advantages relative to conventional subcutaneous drug delivery methods.
However, existing transdermal drug delivery devices often fail to consistently deliver all of the drug beneath the stratum corneum layer of the skin so that it can be absorbed into the body. In this regard, due to the small size of the needles, often times all or a portion of the drug is delivered only onto the top of the skin or into the stratum corneum layer where the drug cannot be absorbed into the body of the patient. This can happen for various reasons. For example, the needle depth may slightly retract from the desired insertion depth such as due to the inconsistent application of force on the needles or the natural elasticity of the skin acts to push the needles outwardly after insertion. Further complicating transdermal delivery with such small needles is that the skin may form such a complete juncture with the needle that the drug flows upwardly along the needle towards the point of insertion and away from the cellular layers capable of absorbing the drug into the body.
Accordingly, there remains a need for a transdermal drug delivery device having an improved ability to consistently and effectively deliver a drug formulation through a patient's skin.